Cooking appliance

ABSTRACT

A cooking appliance using a gas fuel as a heat source and having an appliance body includes comprising a temperature sensor installed in a heating chamber, a control circuit adapted to be actuated by the output from the temperature sensor and to operate according to three working temperatures, namely, a preset temperature, an upper limit temperature and a lower limit temperature, the arrangement being such that when the temperature in the heating chamber being detected by the temperature sensor reaches the upper limit temperature, the main burners are completely closed; when it reaches the preset temperature, the firing of the main burners is reduced by half; and when it reaches the lower limit temperature, the main burners are fully opened, thereby effecting fine temperature control, the firing rate being automatically adjusted according to variations in external conditions affecting the heating chamber temperature, thereby maintaining the preset temperature throughout the heating operation to provide a satisfactory cooking result.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a composite cooking applianceconsisting of a gas oven and a microwave oven, or a cooking appliancesuch as a gas oven, and it particularly relates to a temperature controldevice therefor utilizing gas combustion.

2. Description of the Prior Art

The remarkable advance of semiconductor technology has resulted insophistication of control circuits, miniaturization of such circuits byhigher integration, and reduction of the costs of such circuits bymass-production, and these electronic control circuits have come to bealso widely used in household electric equipment.

The technique using intelligence based on this electronic control ismaking rapid inroads into various heating apparatuses including electricovens, microwave ovens, gas ovens, and combinations thereof.

One of the most important factors in cooking appliances based on gascombustion is exact temperature control. Since the quality of cookingdepends on temperature, it is important that a preset temperaturesuitable for a given heating load be maintained accurately. Conventionalcooking appliances using gas have temperature control means, most ofwhich have been liquid expansion control methods and bimetals.

A gas oven will be taken up by way of example. There is an arrangementwherein a main burner is fired to produce hot air, which is fed into theheating chamber of the oven to cook a heating load therein. Theoperation of a gas oven of this arrangement will be described withreference to FIGS. 4 and 5. When the user sets the knob at a certaintemperature, two temperatures, upper and lower limit temperatures T₁ andT₂, are set by a control circuit. When cooking is started, the mainburners start firing and eventually the upper limit temperature T₁ willbe reached at time t₂. Then, the main burners stop firing, so that thetemperature in the oven heating chamber gradually lowers until the lowerlimit temperature T₂ is reached at time t₃. Then, the main burners startfiring again, repeating this cycle henceforth. If the preset temperatureis changed to a higher one after passage of time t₇ (the upper and lowerlimit temperatures being T₃ and T₄, respectively), the upper limittemperature T₃ is reached at time t₈, as shown in FIG. 4, and henceforththe same operation as described above is repeated.

Thus, the conventional gas oven is designed to control temperature bythe on-off operation of the main burners to maintain the oven heatingchamber temperature at a preset value, but this design has the followingdrawbacks.

The pressure of household gas differs with districts. Even in the samedistrict, the gas pressure available for the gas oven installed in ahome will always very owing to the turning on and off of gas in otherhomes or in another room in the same home. Thus, if the gas pressuredrops below the normal value, this decreases the rate of heat generationby the main burners, thus requiring a longer time than usual to reachthe preset temperature and hence a longer cooking time.

Further, since the ambient temperature of the air surrounding the gasstove differs greatly between midsummer and midwinter, the cooking timewill be different in midsummer, particularly in a drafty room.

Furthermore, because of the design to which turns off all of the mainburners upon reaching of the upper limit temperature and turns on all ofthe main burners upon reaching of the lower limit temperature, theactual difference in temperature between the two extremes is as great asabout 10° C., showing that this design fails to attain the purpose ofmaintaining a temperature at a constant value. Furthermore, the need forfrequently turning on and off the main burners entails the drawback ofshortening the life of the control circuit system, particularly therelay.

SUMMARY OF THE INVENTION

The present invention eliminates such conventional drawbacks.

Accordingly, an object of the invention is to provide a heatingappliance having an automatic control function, wherein a controlcircuit automatically compensates for external conditions which affectthe gas oven heating chamber temperature. More particularly, it providesa cooking appliance comprising a heating chamber for receiving a heatingload, a heating means for heating said heating chamber, a temperaturesensor for detecting the temperature in said heating chamber, and acontrol means including a microcomputer for controlling the output andheating time of said heating means in response to signals from saidtemperature sensor, wherein temperatures detectible by said temperaturesensor are classified into at least three levels, namely, upper limit,middle and lower limit temperatures according to a preset heatingtemperature and the output of said heating means is changed stepwiseaccording to the temperatures at the respective levels so as to controlthe heating chamber with respect to said preset heating temperature. Itis so designed that if the capacity of a main burner in operation isinsufficient for a preset temperature, another main burner is fired soas to maintain the preset temperature as constant as possible, and whenthe preset temperature is attained, a minimum number of main burners areoperated intermittently or in an on-off manner to maintain thetemperature at the preset value.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of a cooking appliance according to anembodiment of the present invention;

FIG. 2 is a layout view of said cooking appliance;

FIG. 3 is a block diagram for explaining the operation of amicrocomputer incorporated in said heating appliance;

FIGS. 4 and 5 are a detection temperature characteristic diagram and acombustion heat generation rate characteristic diagram of a conventionalcooking appliance;

FIGS. 6, 7 and 8 are a detection temperature characteristic diagram, acombustion heat generation rate characteristic diagram, and a heatingchamber temperature characteristic diagram in the "strong" state of acooking appliance according to an embodiment of the invention;

FIGS. 9, 10 and 11 are a detection temperature characteristic diagram, acombustion heat generation rate characteristic diagram and a heatingchamber temperature characteristic diagram in the "medium" state of saidappliance;

FIGS. 12, 13 and 14 are a detection temperature characteristic diagram,a combustion heat generation rate characteristic diagram and a heatingchamber temperature characteristic diagram in the "weak" state of saidappliance.

DESCRIPTION OF THE PREFERRED EMBODIMENT

An embodiment of the invention will now be described. A cookingappliance body 1 shown in FIG. 1 represents a composite cookingappliance comprising a combination of a microwave oven and a gas oven,but only the gas oven section will be taken up for explanation.

In FIG. 2, turning a gas cock knob 8 opens a gas cock 9 andsimultaneously turns on a gas cock switch 10, actuating a high voltagespark generator 11 to cause an ignition plug 12 to produce a spark. Onthe other hand, the gas entering through a hose end 13 passessuccessively through the gas cock 9, a now-opened main solenoid valve14, a governor 15 and a pilot gas passage 16, and enters a pilot burner7. The latter is ignited by said spark and a thermocouple 17 is therebyheated, signaling a microcomputer 18 to open first and second solenoidvalves 4 and 5. The gas flows through main gas passages 19 and 20 intomain burners 2 and 3, where it is ignited by said pilot burner 7.Henceforth a temperature sensor 6 detects the temperature in the heatingchamber (not shown), signaling the microcomputer 18 to turn the firstand second solenoid valves 4 and 5 on and off so as to control the ovenheating chamber temperature to maintain it at a preset heatingtemperature.

The configuration of the microcomputer 18 or of FIG. 2 is shown in FIG.3. The output from the temperature sensor 6 and the output from atemperature setting circuit 22 which has been preset by the user arecompared in a comparator circuit 23. If the temperature detected by thetemperature sensor 6 is higher than the preset temperature, thecomparator circuit 23 actuates a driver circuit 24 to cut off the secondsolenoid valve 5. As a result, one main burner 3 stops firing.

If the temperature further rises by about 3° C., a shift circuit 25 isactuated to cause a comparator circuit 26 to make a comparison betweenthe output from the temperature sensor 6 and the output from thetemperature setting circuit 22. If the temperature detected by thetemperature sensor 6 is higher, the comparator circuit 26 actuates thedriver circuit 24 to cut off the first solenoid valve 4. As a result,the other main burner 2 stops firing.

Cooking in the High Temperature Region

FIGS. 6-8 illustrate a situation where the heating chamber temperatureis set in the 250°-300° C. range which is a high cooking temperatureregion for cooking loads including fish and sweet potatoes which require300° C. In the case where the preset temperature is 300° C., differentvalves of heating chamber temperature are indicated by Rm₃, (Rm₃ +about3° C.) by Rh₃, and (Rm₃ -about 3° C.) by Rl₃. Similarly, temperaturesdetected by the temperature sensor 6 in the case of said presettemperature of 300° C. are indicated by the middle temperature Tm₃,upper limit temperature Th₃ and lower limit temperature Tl₃. The presettemperature is established by the user adjusting the knob to 300° C.;thus, three temperatures are designated, two of which are shifted 3° C.above and below said temperature of 300° C. In order to detect heatingchamber temperatures more accurately it is so arranged that thetemperature sensor 6 will detect them at points about 5° C. higher. Thisis because there is a discrepancy between the heating chambertemperature and the temperature detected by the sensor 6. That is:

Middle temperature Tm₃ =Rm₃ +about 5° C.

The same is true of the upper limit temperature Th₃ and the lower limittemperature Tl₃. Detection of temperatures by the temperature sensorprovides on-off control of the main burners 2 and 3, whereby the heatingchamber temperature is adjusted. Where the pilot burner 7 is firing atall times, let the combustion heat generation rate be indicated by C₂Kcal/h when the two main burners 2 and 3 are all fired, by C₁ Kcal/hwhen the main burner 2 alone is fired, and by C₀ Kcal/h for the pilotburner 7 alone.

In the initial stage of combustion, the main burners 2 and 3 are fired,rapidly heating the heating chamber with the combustion heat generationrate of C₂. Upon lapse of t₁ minutes, when the temperature beingdetected by the temperature sensor 6 reaches the middle temperature Tm₃° C., the microcomputer 18 turns off the second solenoid valve 5,putting out the main burner 3. At this time, the main burner 2 remainsfiring. Thus, the combustion heat generation rate lowers to C₁ Kcal/h.However, the heating chamber temperature drops after it has overshot orsome time owing to the remaining heat. When the temperature beingdetected by the temperature sensor 6 reaches the lower limit temperatureTl₃, the microcomputer 18 turns on the second solenoid valve 5, ignitingthe main burner 3. Thereafter the aforesaid control is repeated untilcompletion of cooking.

Thus, when the temperature being detected by the temperature sensor 6reaches the middle temperature Tm₃ ° C., one of the two burners, or theburner 3, is automatically put out, and it depends on the value of thepreset temperature whether the heating chamber temperature furtherrises, remains as it is, or drops. More particularly, in the case ofcooking in the high temperature region around 300° C., stopping oneburner 3 results in the heating chamber temperature tending to lower ata rate dependent on the heating chamber heat capacity and burnercapacity, until it reaches the lower limit temperature Tl₃. Thereupon,the control circuit is actuated again to ignite the previously stoppedmain burner 3. Since the other main burner 2 remains firing during thisperiod of time, the temperature in the heating chamber is kept high,thus eliminating the drawback of the conventional control means causinga large difference in temperature owing to the fact that all the burnerssimultaneously turn on and off repeatedly. Theoretically, the size oftemperature change is half that for the conventional control means.

It is so arranged that if the door 21 should be opened in the course ofcooking, as at t₅, the flames of the main burners 2 and 3 will be putout for safety and the hot air circulation fan (not shown) will bestopped; but this will rapidly lower the temperature in the heatingchamber. The door 21 is closed and heating is restarted, when thetemperature in the heating chamber is S'° C., which is below the lowerlimit temperature Rl₃, while the temperatures S° C. detected by thetemperature sensor 6 at this time is also below the lower limittemperature Tl₃. As a result, the main burners 2 and 3 are ignited. Thehot air circulation fan is also operated again. The normal control willbe repeated henceforth.

In FIG. 6, if the temperature sensor 6 detects the upper limittemperature Th₃ for some reason or other (which detection is notillustrated), the main burners 2 and 3 are put out, with the pilotburner 7 alone firing.

As described above, in the "high" range between 250° C. and 300° C.,three detection levels are assigned to the temperature sensor 6 todetect the lower limit, middle, and upper limit temperatures. The twomain burners 2 and 3 are used in the initial stage of heating and whenthe middle temperature Tm₃ is reached, the main burner 3 is put out,with only one being used for heating. When the lower limit temperatureTl₃ is reached, both burners take part in heating, but when the upperlimit temperature Th₃ is reached, both of the main burners 2 and 3 areput out, with the pilot burner 7 alone firing. The aforesaid detectionlevels of the temperature sensor 6 are stored in the microcomputer 18 inadvance, and when the preset temperature value is inputted into themicrocomputer, the optimum upper limit, middle and lower limittemperatures are selected and control is effected.

Cooking in the Middle Temperature Region

A description will be given of a manner of control which is effectedwhere the preset heating temperature is in the "middle" range of about200°-250° C. As shown in FIGS. 9-11, the two main burners 2 and 3 areused in the initial stage of heating, and when the temperature beingdetected by the temperature sensor 6 reaches the middle temperature Tm₂,one main burner 3 is put out. Upon lapse of t₆ minutes during whichovershooting takes place, the lower limit temperature tl₂ is reached,whereupon the main burner 3 is ignited. The control continues with thisoperation repeated, but when the upper limit temperature Th₂ is reached,both main burners are put out.

Thus, in the case where the preset temperature is in the middle region,the temperature in the heating chamber will not change so much even ifone main burner 3 is put out, thus allowing the other main burner 2alone to continue firing. If such a combustion state proceeds until theupper limit temperature Th₂ or lower limit temperature Tl₂ is reached,the control circuit performs the same control operation as describedabove to maintain the preset temperature.

Cooking in the Low Temperature Region

A description will be given of a manner of control in the case ofcooking where the preset heating temperature is in the "low" range ofabout 150°-200° C. As shown in FIGS. 12-14, in a preheating period fromthe start to time t₂, both of the two main burners 2 and 3 are fired,but when the temperature being detected reaches the preset middletemperature Tm₁, one main burner 3 stops firing. However, since thecombustion capacity of one burner is large as compared with the size ofthe heating chamber, the remaining one burner 2 in operation issufficient for the heating chamber temperature to keep on rising untilat time t₃ it reaches the upper limit temperature Th₁. At this point,the main burner 2 is also put out; that is, none of the two main burners2 and 3 are firing, with the pilot alone firing. As a result, thetemperature lowers, and at time t₄ the middle temperature Tm₁ is reachedwhereupon one main burner 3 is fired again. Henceforth such an on-offoperation is repeated. In this case also, since it is not all mainburners but only one main burner 3 that is on-off operated, thetemperature change is small.

In the case of cooking in the low temperature region, normally thetemperature in the heating chamber is on the increase even after onemain burner 3 is turned off, but under special conditions as when thegas pressure is so low that the use of a single burner alone isinsufficient or when the ambient temperature is extremely low as inmidwinter, if one main burner 3 is turned off, the heating chambertemperature will soon lower to the lower limit temperature Tl₁. In thiscase, therefore, one main burner 3, now put out, is ignited again tokeep the temperature rising. This operation is the same as the one thatwas described with reference to cooking in the high temperature region;thus, even if the setting is in the low temperature region, the controlcircuit will automatically come into operation depending upon a changein the external conditions including gas pressure and ambienttemperature, so as to provide the optimum conditions for maintaining thepreset temperature.

Operations similar to the one described above will be performed in thecase of cooking in the medium and high temperature regions. In addition,the number of main burners to be used may be optionally determined, asoccasion demands.

As is clear from the foregoing description, the present embodimentassigns three levels of detection to the temperature sensor 6 fordetecting the upper limit temperature, middle temperature and lowerlimit temperature, wherein upon detection of the upper limittemperature, all main burners are turned off; upon detection of themiddle temperature, the combustion heat generation rate of the mainburners is reduced approximately by half; and upon detection of thelower limit temperature, all the main burners are turned off. Thus, thefollowing effects are obtained.

1. Generally, the flow rate varies to a large extent with the types ofthe gas, nozzle and governor, and in the case of coal gas, thecalculated flow rate decreases by about 40 percent at worst, whichaccounts for the fact that with the conventional heating control system,the combustion heat generation rate is insufficient, leading to afailure in cooking or to protraction of the cooking time. In contrast,in the present embodiment, the microcomputer control automaticallycompensates the combustion heat generation rate for variations in theinput gas flow rate, so that the proper combustion heat generation ratecan be maintained for any preset temperature.

2. Since the temperature sensor, providing selective use of three levelsof heating power, makes it possible to finely control heating, the sizeof variations in the heating chamber temperature is small, ensuringsatisfactory results of cooking.

3. Since full heating power is developed in the initial stage ofheating, the initial rise of temperature is quick. Further, even if thedoor is opened during heat, the initial re-rise of temperature is alsoquick because of the development of the full heating power.

4. During heating for initial rise of temperature one of the mainburners is put out when the central temperature Tm is reached, so thatless overshooting takes place and no preheating is required and hencethe cooking time is shortened.

5. Since the frequency of on-off actions of the solenoid valve isreduced, the durability of the solenoid valve is improved and low-noiseoperation is possible.

6. Complex control in multistage, such as three stages "strong, medium,weak," is made possible by computer control.

As has been described so far, according to the present invention, anypreset temperature can be automatically compensated for insufficient gasflow rates dependent on the type of the gas or due to various troublesto the piping, and sufficient gas flow rates can be attained. Further,the main burners are controlled so that they develop their full power,about half the full power or zero power when the temperature beingdetected by the temperature sensor reaches one of the at least threetemperatures, namely, the upper limit temperature, middle temperatureand lower limit temperature, no matter what the preset heatingtemperature may be. Thus, it is possible to provide a cooking appliancecausing smaller size of variations in the heating chamber temperature,thus ensuring satisfactory results of cooking.

We claim:
 1. A cooking appliance comprising: a heating chamber forreceiving a heating load, a heating means for heating said heatingchamber, an input means for providing a preset temperature input, atemperature sensor for detecting the temperature in said heatingchamber, and a control means including a microcomputer for controllingthe heat output and heating time of said heating means in response tosignals from said temperature sensor and said input means, whereintemperatures detectible by said temperature sensor are classified bymeans contained within said microcomputer into at least three levels,namely, an upper limit temperature, a middle temperature, and a lowerlimit temperature according to said preset heating temperature andwherein said heat output of said heating means is changed in a stepwisefashion according to said temperatures at the respective levels so as tocontrol said heating chamber with respect to said preset heatingtemperature; and wherein when said temperature being detected by saidtemperature sensor is respectively said upper limit temperature, saidmiddle temperature and said lower limit temperature, said heating meansdevelops no heat output, half its full heat output and its full heatoutput, respectively, thereby controlling the atmosphere in said heatingchamber so as to be at said preset heating temperature.
 2. A cookingappliance as set forth in claim 1, wherein in an initial stage ofheating, said heating means develops its full output, but it developshalf of its full output when said middle temperature is first reachedand henceforth it varies its output with respect to said detectedtemperature, thereby controlling the atmosphere in the heating chamberso that it is at said preset heating temperature.